Referring to FIGS. 1 and 2, a conventional wireless power transfer system transfers power through resonant inductive coupling, and includes a wireless power transmitter and a wireless power receiver. For the wireless power receiver, when a switch (SW1) conducts while another switch (SW2) does not conduct as shown in FIG. 1, an AC (alternating current) current output (Is) provided by a receiver resonator has a peak amplitude that is greater than zero as shown in FIG. 3 and that decreases gradually (not shown), and a DC (direct current) current output (Ir) provided by an AC-to-DC converter 9 to a load has a peak magnitude that is greater than zero as shown in FIG. 3 and that decreases gradually (not shown); and when the switch (SW1) does not conduct while the switch (SW2) conducts as shown in FIG. 2, the peak amplitude of the AC current output (Is) is greater than zero as shown in FIG. 3 and increases gradually (not shown), and the peak magnitude of the DC current output (Ir) is zero as shown in FIG. 3 (i.e., no current).
The AC current output (Is) has a relatively large root mean square amplitude when the AC-to-DC converter 9 provides no current to the load, resulting in relatively large power loss of the wireless power receiver and relatively low power conversion efficiency of the wireless power receiver. Moreover, the AC current output (Is) flows back and forth through the conducting switch (SW2), so body switching technique has to be used on the switch (SW2) when the switch (SW2) is a MOSFET (metal oxide semiconductor field effect transistor) switch.